In existing structures of secondary batteries and battery packs, the use and arrangement of secondary batteries may be limited. Particularly, a battery pack is generally configured with quadrangular batteries having the same capacity, size and dimension. The physical shape of a secondary battery reflects the electrical arrangement of the secondary battery. For example, a battery pack having six electrode assemblies may include six electrode assemblies having the same size and capacity. In the battery pack, the electrode assemblies may be parallelly provided in two rows—each having three electrode assemblies. However, this general type of secondary battery or battery pack may not be effectively used in a space having a non-quadrangular or irregular shape for a portable electronic device or a large capacity vehicle battery. Therefore, a plurality of electrode assemblies having different shapes from each other may be provided in order to effectively use the available space. In this case, the output thereof may not be constant due to the difference in capacity of each electrode assembly.